Nickel (Ni) based superalloys are used as materials in applications that are exposed to high temperatures for long periods. Examples of such applications are gas turbine engines, where a reasonable strength at high temperatures, stability of microstructure, oxidation resistance, and low density of the materials are important considerations. Such Ni-based superalloys can be used in a wrought form, or consolidated form from powder, or in a cast form.
In an example, wrought alloys include IN718 described in U.S. Pat. No. 3,046,108. The nominal composition of the alloy described in this invention is Ni-17% Fe-19% Cr-3.15Mo-5.15% (Nb+Ta)-5% Al, and it has a density of 8.19 gm/cm3. This alloy retains its yield strength, which is 950 Mega Pascal (MPa) at about 650° C. in large sections. Further, the alloy is strengthened by a fine distribution of intermetallic precipitates based on the intermetallic compound, Ni3Al, designated as γ′ and an intermetallic compound based on Ni3Nb in a matrix containing Ni and Fe (designated γ″). Another commonly used wrought alloy used in disc and blade applications of gas turbine engines, UDIMET 720 with a nominal composition Ni-16% Cr-14.75% Co-3% Mo-1.25% W-5% Ti-2.5% Al-0.01% C-0.0275% Zr-0.15% B and a density of 8.08 gm/cm3 by weight, was introduced in 1986 (UDIMET is a Registered Trade Mark of Special Metals Corporation), and has a slightly improved strength at 700° C.
An alternative approach for large high temperature applications is based on consolidation of alloy powders. The usage of powder permits higher alloying levels without attendant segregation of elements in the product. For example, the U.S. Pat. No. 5,104,614 discloses the composition of alloy designated as N18 which retains high temperature yield strength to about 1000 MPa at 750° C. The use of powder permits higher alloying levels without attendant segregation of elements in the product. The nominal composition of N18 is Ni-11.5% Cr-15.7% Co-6.5% Mo-0.6W-4.5% Al-4.35% Ti-0.45Hf by weight and this alloy is strengthened by a fine dispersion of the intermetallic compound γ′ in a matrix of γ.
Further, in certain high temperature applications, cast alloys are used. Such alloys may be in the equiaxed, directionally solidified or single crystal form. For example, U.S. Pat. No. 5,366,695 describes a single crystal composition, known commercially as CMSX10, which is nominally Ni-5.7% Al-0.2% Ti-2% Cr-3% Co-5% W-8% Ta-6% Re-0.4% Mo-0.33% Hf by weight and has a density of 9.05 gm/cm3. This alloy is also precipitation-strengthened by high volume fractions of γ′ and retains high temperature yield strength of about 950 MPa to temperatures greater than 850° C. The process used to manufacture a material using the cast alloys limits its application to manufacturing of products having thin sections and limited dimensions. The materials represented by this class of alloys are used as turbine aerofoils in gas turbines. Further examples of such single crystal alloys strengthened by high volume fractions of γ′ can be found in U.S. Pat. No. 6,966,956 and US Patent Publication No US2010/0143182, which teach improvement of the long term stability and oxidation resistance of such alloys.
Alloys as described above are derived from the beneficial effects of a fine dispersion of an intermetallic compound Ni3Al in a disordered matrix strengthened by various elements that contribute to solid solution strengthening and also limit atomic mobility at high temperatures. Such γ′-strengthened cast alloy compositions strengthened cast alloy compositions can be reinforced by unidirectionally aligned, coarsely spaced, carbide fibers through directional solidification of eutectic compositions, as described, for example, in U.S. Pat. No. 3,904,402. These materials retain strength levels of about 950 MPa to nearly 870° C. Similarly, U.S. Pat. No. 4,111,723 discloses another example directionally solidified eutectic alloy with molybdenum fibers. Due to a unidirectional alignment of fibers, the properties of these directionally solidified eutectic alloys are anisotropic, with poor transverse properties. Further, the process of manufacture of such eutectic alloys requires low solidification rates, which may lead to long manufacturing cycles.
Ni based alloys have emerged as materials of choice for high temperature applications in the range 600° C. to 1110° C. based on the properties of the disordered matrix γ phase and the intermetallic compound γ′ of the Ni based alloys. However, the high temperature properties of such Ni based alloys are ultimately limited by presence of the disordered matrix γ phase. U.S. Pat. No. 5,336,340 discloses a different metallurgical approach consisting of a combination of the intermetallic compound Ni2AlTi (β′) and Ni3Al (γ′) dispersed in a matrix of the intermetallic compound NiAl (β) in the Ni—Ti—Al system. Such alloys are shown to possess extremely high strength, ranging from 1000 MPa to 1455 MPa at room temperature and retain high strength up to 1200 MPa at 700° C. However, the alloys have only been tested in compression and no evidence of tensile ductility, which is important for engineering applications, has been provided.
It is known that finer scale structures arising from eutectic or peritectic reactions in iron, magnesium, titanium and aluminium alloys with disordered matrix phases and intermetallic compounds offer improved combinations of compressive strength and ductility. However, it is also well known in the prior art that intermetallic compounds offer improved high temperature properties but suffer from the problems of ambient temperature ductility.
The present invention exploits the interaction between eutectic and peritectic reactions that form intermetallic compounds, including γ′ in certain binary systems with Ni as the base, to form fine scale structures constituted entirely of different combinations of intermetallic compounds in ternary and more complex systems.